As a power amplifier for amplifying a PWM signal, known in the prior art is a so-called class D amplifier. Since such class D amplifier has a transistor at its output stage which can operate in a switching mode, the output transistor produces less heat and hence, it is generally used as a power amplifier which is small in construction but produces a large output. In general, since if a switching frequency is higher, the efficiency becomes higher and higher, for the switching transistor, is used a field effect transistor which can operate in high speed.
But, the field effect transistor for use in power amplification has a defect such that it becomes more expensive than a bipolar transistor.
On the other hand, since the bipolar transistor can not set its switching frequency as high because of its carrier concentration effect, it has also such drawback that is can not carry out the efficient power amplification.